power gains with cat bypass pipes

[Johnny Danger]

Originally Posted by The Radium King

i will agree that the throttle response on my '00 S feels 'disconnected'; i've always attributed that to the e-gas. further, any improvement in throttle response does not necessarily mean that you are making more hp/torque. your car sounds like it is a lot for fun to drive, however.

to jd i would ask (a) the hp increase was where in the rpm range, and (b) any hp effect that a cat bypass pipe is +/- say 5 hp on a 200 hp vehicle; can any of us say that we can 'feel' a 2.5% change? there is a larger variation between two stock engines, or two engines at two different altitudes, or a hot vs cold engine, or a driver with a fat girlfriend vs a driver with a skinny girlfriend. and what effect does the audio component have on how you feel the power response (ie, it 'sounds' more powerful; the 'sound' makes you want to rev higher)?

all i know is what i've seen, and all dynos of aftermarket exhaust show a minor torque reduction at lower rpm (if they show lower rpm at all; often the dynos start at 3000 rpm).

not being argumentative; just being conversational and trying to distill the facts given the sometimes difficult nature of electronic conversation.

trk.

My "journey" toward creating a better exhaust system for my vehicle began with the installation of an aftermarket cat back exhaust. My first impression was, that this upgrade produced a much more uniform engine response, and of course better sound. Subsequently, I deleted and by-passed the secondary cats. Again, the vehicle just felt "better", with no sense of loss with regard to low or mid range torque. It wasn't until I began experimenting with the header/pre-cat setup that I experienced varying results. To make a long story short, the design that ultimately lead to the best results for me was a set of custom fabricated headers that incorporated longer, equal length primaries that flowed into a well devised merger collector - followed by a set of 200 cell tri-metal HJS cats. Dyno results later showed consistent gains in hp through out the entire power curve.

[The Radium King]

ok. most dynos i've found are for (a) headers, and (b) cat back (ie, both mid pipes and muffler). again, very few perform on paper.

if we look at the variation in cats between usa and row cats, we could assume that the secondary cats on usa cars are an afterthought (required for emissions) and were not considered in the initial tuning of the exhaust. so, we can return to the intended tune of the car by going to catless mid pipes (as well as reduce some lbs and improve sound - cool).

if that assumption is true, then the meaningful tuning comes from the headers and the relevant back pressure comes from the exhaust. i have a set of ebay 'knock-off' headers and would note that, while they are presumeably a copy of fabspeed or somesuch, they are very rough at the manifold flange and could really benefit from an old-school polish. as you state, varying results. a longer header will reduce the negative effect of equal length primaries at low rpm, but also reduce the benefits of tuning for low rpm. it gets down to fuzzy lines on inaccurate dynos. seat of the pants is your best measuring stick.

[Johnny Danger]

Originally Posted by The Radium King

ok. most dynos i've found are for (a) headers, and (b) cat back (ie, both mid pipes and muffler). again, very few perform on paper.

if we look at the variation in cats between usa and row cats, we could assume that the secondary cats on usa cars are an afterthought (required for emissions) and were not considered in the initial tuning of the exhaust. so, we can return to the intended tune of the car by going to catless mid pipes (as well as reduce some lbs and improve sound - cool).

if that assumption is true, then the meaningful tuning comes from the headers and the relevant back pressure comes from the exhaust. i have a set of ebay 'knock-off' headers and would note that, while they are presumeably a copy of fabspeed or somesuch, they are very rough at the manifold flange and could really benefit from an old-school polish. as you state, varying results. a longer header will reduce the negative effect of equal length primaries at low rpm, but also reduce the benefits of tuning for low rpm. it gets down to fuzzy lines on inaccurate dynos. seat of the pants is your best measuring stick.

For the most part I agree with your assessment. Upgrading the oem system involves a delicate balance of reducing restricted air flow, and at the same time maintaining a necessary amount of back pressure . I found this to be especially true throughout my experimentation with headers/pre-cats. Ultimately, the most successful application for me involved following the basic design principles of the oem lay-out, however, improving upon them in the ways previously mentioned. The quality of materials, smoothness of welds, precise mandrel bending and the particular elements used in the cats were all extremely crucial to the overall success of my exhaust project . Once this was accomplished, the end result yielded an impressive increase in performance .

[jonnycool]

I have a pair of Janspeed decat pipes...in the garage! I 've had them for around 18 months but I've been a little torn between fitting them or not, not because I'm worried about back pressure or noise I'm more concerned about the legality of it.

Ok I'm sounding like an old fart I know but you get rear ended at the traffic lights, it's not your fault but are you now still insured with your lovely sounding decat pipes? You can't tell your insurance about your mod to get it covered because they are not meant to be on there in the first place.

I paid nearly £300 for mine but then got the jitters when it came to fit them, sport cats may be a better way to go like the blue flame cats at £400 the pair?

It may not apply to wherever you live but I just thought it was worth mentioning just in case it results in tears for any forum members.

[jaykay]

I have a by-pass modified OEM muffler which physically limits flow into the muffler. As the flow rate increases more and more gas volume is force out the by-pass route as turbulence starts to block up the entry into the muffler. This is the theory anyway. I have also dropped my secondary cats(fabspeeds) but still have the stock headers. It is hard to tell but I think I have lossed low end torque and of course all hell breaks loose above 4K.

The sound is truly amazing but can give you a false sense of performance.... I often wonder if I have hurt the performance of the car. Perhaps I have too little back pressure for proper scavenging in certain regimes.

Also wonder what ECU flash would work best with this set-up.